The present invention relates to a technique for modulating a digital signal.
It therefore applies to the transmission art, in particular the radio transmission art.
A typical application is aimed at radio systems, in particular “broadband” systems. These systems are naturally designed to offer a high capacity and it is therefore necessary to use a modulation technique that offers high spectral efficiency, which amounts to saying that the spectrum of a given channel must be as narrow as possible consistent with its technical specifications.
Moreover, constant envelope modulation is generally employed, which minimizes the complexity of transmitters. If the signal features variations of relatively large amplitude, the amplification stages must be perfectly linear, especially the power amplifier.
It nevertheless appears to be the case that all forms of constant envelope modulation known in the art until now have a spectrum which features a side lobe.
If it is within the spectrum of a neighboring channel, the side lobe increases the level of interference in that neighboring channel.
The Gaussian minimum shift keying (GMSK) modulation technique employed in the GSM has a −40 dBc side lobe at 200 kHz from the main lobe. The spacing between two adjacent channels is also 200 kHz. Obviously this reduces spectral efficiency.
The spectrum of quadrature amplitude modulation (QAM) has no side lobe if an appropriate filter is used.
However, these modulation techniques cause strong variations in the amplitude of the modulated signal. As mentioned above, it is necessary to use more complex and therefore more cost amplifiers in this case.